Singleton compilation error LNK2001

[bonho]

I'm trying Singleton to generate unique number during a session and arrived at the error

Error 1 error LNK2001: unresolved external symbol "private: static class IDGenerator * IDGenerator::only_copy" (?only_copy@IDGenerator@@0PAV1@A) C:\Users\Bonaventure\Documents\Visual Studio 2013\Projects\Test\Test\Test.obj Test


Within "EventDef.h"

#ifndef EVENT_DEFINTIONS_H
#define EVENT_DEFINTIONS_H

class IDGenerator {
private:
	IDGenerator() : _id(0) {}

	static IDGenerator* only_copy;
	long _id;
public:
	   static IDGenerator* instance() {
		   if (!only_copy) {
			   only_copy = new IDGenerator();
		   }
		   return only_copy;
	   }
	   long next() { return _id++; }
};

#endif 

Test.cpp

#include "EventDef.h"

void main() {
  long num = IDGenerator::instance()->next();
}

[TwilightSpectre]

Because it is a static instance, you need to initialize it in another CPP file as well.

Also, why do you need this just to generate unique ID's? This is all you need:

unsigned long IDGenerator() {
    static unsigned long id = 0;
    return id++;
}

[bonho]

oh great that's even better thanks.